There is a KNIME Labs plug-in named KNIME Datageneration (http://tech.knime.org/datageneration). It gives support to generate values from different distributions for existing rows to new columns:
To generate rows with numeric content, the most obvious node is Data Generator. It generates data for clusters of normally distributed data for various dimensions with different cluster centers on the
[0,1] interval. It also generates the cluster labels.
To generate empty rows for existing tables, the Add Empty ...